1. Field of the Invention
The present invention relates to the production of polymers in gas phase reactors. In particular, the present invention concerns a method and apparatus for producing olefin polymers in gas phase reactors substantially in the absence of chunks and sheets.
2. Description of Related Art
Gas phase polymerization reactors generally comprise an elongated reactor body having an essentially vertically disposed central axis. The reactor is divided into two distinct parts, viz. an upper part and a lower part, which are separated by a distribution plate. Polymerization takes place in the upper part of the reactor body, in a fluidized bed formed by catalyst particles. The lower part is a mixing zone. Thus, one or several monomers or a gas containing the monomer(s) are fed into the reactor through an inlet placed in the lower part of the reactor by using a compressor or blower. The inlet comprises, for example, feed nozzles, which are usually provided with deflector means for achieving intensive mixing of the feed gas within the lower part of the reactor. From this mixing zone, the gas flow is conducted through the distribution plate, which aids in uniformly distributing the monomers into the fluidized catalyst bed above the distribution plate. The distribution plate comprises a plurality of openings, which conventionally are covered with (over)caps to provide sideward directed flow of the gas. The monomers are polymerized on the catalyst particles, ideally forming particulate polymer granules.
Unreacted monomers or monomer-containing gas are withdrawn from the reactor through an outlet located in the upper part of the reactor, above the surface of the fluidized bed. The unreacted gas is usually cooled and recycled to the reactor inlet. Polymer particles are separately withdrawn from the reactor and subjected to further treatment, e.g. to a second polymerization step for producing homo- or copolymers or to degasification and pelletizing to produce homo- or copolymer pellets.
In conventional gas phase reactors of the above kind, there is one single distribution plate inside the reactor body. The fluidized bed is formed above the distribution plate and does not extent below it. The plate separating the upper and the lower parts will retain a significant part of the polymer particles in case of a shutdown.
Chunk and sheet formation is a problem in conventional gas phase reactors. The chunks and sheets tend to make the flow of monomer gas through the fluidized bed less homogeneous giving rise to pressure drop and clogging. They may even plug the outlet. Eventually, they will have to be removed from the reactor, which impairs productivity.
Chunks and sheet are usually formed in stagnant zones within the reactor. For example, chunk formation has also been observed at the overcaps above the openings of the gas distribution plate. In the art, certain improvements of the construction of the gas distribution plates have been proposed.
EP-A-963 786 discloses a gas distribution plate having no caps over the openings. The openings are straight passages, with a conical outlet at the upper part of the plate.
GB-A-2 271 727 discloses a gas distribution plate having a specific arrangement of openings. The openings are arranged at the vertices of the random squares adjoining to one another on the plate, and also at positions close to the sidewall of the reactor. The openings are covered with caps.
EP-A-721 798 discloses a gas distribution plate where the openings at the outer peripheral part of the plate have larger diameter that the openings at the inner peripheral part. The openings are preferably provided with caps.
Although the above distribution plate constructions represent, generally speaking, an improvement of the distribution of the gas within the fluidized bed, experience shows that chunks and sheets are still formed in gas phase reactors even if they are provided with the above-described distribution plates.
GB-A-1 014 205 describes a fluidisation reactor of a different kind than the one identified above in the introduction, namely a reactor having a plurality of gas distributor plates located at different levels inside the reactor body. The openings in the gas distributor plates are first constricted in the direction of gas flow and then widened so that the total area of the openings at the narrowest points is equal to 30 to 70% of the total cross-sectional area of the fluidization reactor. The openings are e.g. formed by concentric apertures having an annular shape.
Since there are several distributor plates inside the reactor, flow conditions will be significantly different from those in a reactor of the present kind. The construction shown in GB 1 014 205 is quite complicated, as it consists of separate rings, which need to be supported over the whole diameter of the reactor.